Since blue light emitting diodes prepared by using a nitride semiconductor have been developed, requirements to a rise in the performances of light emitting diodes are rapidly increasing. At present, gallium nitride (GaN) is suitably used as a material for light emitting diodes and semiconductor lasers which emit blue light, and it has been important to further enhance the performances of GaN devices.
In general, in order to obtain nitride semiconductor devices having such excellent device performances that the emission output is high and that dispersion of an emission wavelength is small, it is important to provide a high-quality nitride crystal itself constituting the device. Nitride semiconductor devices have so far been produced by allowing a nitride crystal to grow heteroepitaxially on a single crystal substrate such as sapphire to form a nitride crystal body. In order to form a high quality nitride crystal, the lattice mismatch between a substrate and a nitride crystal has to be reduced in order to inhibit forming of a crystal defect of the nitride crystal. A method in which an aluminum nitride single crystal film as a buffer film is formed on a substrate such as sapphire and a nitride crystal as a semiconductor device is formed thereon is known as a method for reducing the lattice mismatch between a substrate and nitride crystal.
A sublimation method attracts as a promising production technology for an aluminum nitride single crystal film as a buffer film (non-patent documents 1 and 2). The sublimation method is a growing method in which a raw material is sublimated at high temperature to reprecipitate crystal on a cold part. However, when the raw material has a low purity, impurities are sublimated as well. Consequently, the quality of the resulting single crystal film becomes low as well and a nitride crystal which is formed thereon and becomes a semiconductor device is also reduced in quality. Accordingly, a high-quality aluminum nitride is required for a raw material used in the sublimation method.
In aluminum nitride, an oxide layer (alumina component) is unavoidably formed on a surface. Such alumina component causes as well a reduction in a quality of a single crystal film obtained by the sublimation method. Accordingly, it is not necessarily suitable to use an aluminum nitride powder having a large surface area as a raw material used in the sublimation method.
On the other hand, an aluminum nitride sintered body has a small surface area as compared with that of a powder having the same weight, and therefore has a surface oxide in small amount. Accordingly, it is investigated to use the aluminum nitride sintered body as a raw material for growing single crystal by the sublimation method.
Various high-purity aluminum nitride sintered bodies have already been known, though they are not intended to use as a raw material for growing single crystal by the sublimation method (for example, patent documents 1 to 3). However, oxides originating in a sintering aid are contained in high-purity aluminum nitride sintered bodies in many cases. That is, aluminum oxide present in an aluminum nitride powder as a raw material or on a surface of the powder forms composite oxide with the sintering aid and the above composite oxide is precipitated on a grain boundary of an aluminum nitride crystal to result in a sintered body containing oxygen, metal components and the like which constitute the composite oxide. For example, patent document 1 discloses an aluminum nitride sintered body having an oxygen concentration of 0.05% by weight and a rare earth metals concentration of 240 ppm; patent document 2 discloses an aluminum nitride sintered body having an oxygen concentration of 300 ppm and a rare earth metals concentration of 50 ppm and an aluminum nitride sintered body having an oxygen concentration of 500 ppm and a rare earth metals concentration of 0.01 ppm; and patent document 3 discloses an aluminum nitride sintered body having an oxygen concentration of 300 ppm and a rare earth metals concentration of 50 ppm.
Such oxygen and metal components cause as well a reduction in the quality of a single crystal film obtained by sublimation method. Accordingly, when a high-purity aluminum nitride sintered body is used as a raw material for growing single crystal by sublimation method, the amount of composite oxide originating in a sintering aid has to be reduced.
Patent document 1: JP-A 1988-25278
Patent document 2: JP-B 2856734
Patent document 3: JP-B 2829247
Non-patent document 1: G. A. Slack and T. F. McNelly, “AlN single crystals”, J. Cryst. Growth 34(1976) 263
Non-patent document 2: M. Miyanaga at al., “AlN Single Crystal Growth by Sublimation Method”, SEI Technical Review, No. 168 (March, 2006), 103
In burning an aluminum nitride powder, a sintering aid is reacted with oxide contained in the raw material powder to form composite oxide, and therefore an aluminum nitride crystal itself is purified. However, the composite oxide is precipitated on a grain boundary of the aluminum nitride crystal, and therefore a purity of an aluminum nitride sintered body as a bulk does not necessarily become high. Accordingly, an object of the present invention is to provide a high-purity aluminum nitride sintered body by efficiently removing oxides contained in a raw material powder and preventing composite oxide produced by reaction of oxides contained in the raw material powder with a sintering aid from remaining in an aluminum nitride sintered body in producing the aluminum nitride sintered body.
The present inventors have intensively investigated to remove composite oxide from the sintered body and have paid attentions on alkaline earth aluminates as a sintering aid.
It has so far been known that alkaline earth oxides are effective as a sintering aid.
It is considered that, in a sintering step, a sintering aid reacts with aluminum oxide (alumina) contained in an AlN powder to form a liquid phase and to discharge oxygen contained in the AlN powder to a grain boundary of AlN crystal in the form of composite oxide. As a result, the AlN crystal is considered to be purified. Among alkaline earth oxides, particularly, alkaline earth aluminates are considered to have a high reactivity with AlN crystal and to have essentially a high ability to purify the AlN crystal because they can form readily and evenly the liquid phase.
Also, oxygen contained in a raw material powder is considered to be discharged outside AlN crystal in the form of composite oxide with alkaline earth aluminate. The above composite oxide is considered to be more readily volatilized than composite oxide of oxygen with a rare earth oxide auxiliary contained in the raw material powder.
Further, alkaline earth aluminate is liable to be volatile as compared with a rare earth oxide auxiliary. Therefore, it is considered that the alkaline earth aluminate itself is also liable to be removed outside an AlN sintered body at a later stage of a sintering step of AlN so that oxygen originating in the alkaline earth aluminate is less liable to remain in the sintered body.
However, it is considered to be difficult for the alkaline earth aluminate to remains in AlN crystal for a sufficiently long time to thereby trap oxygen from the crystal since the alkaline earth aluminate is liable to be volatile.